"We don't know if it was a steam burn or a metal burn, but he was burned over 80 percent of his body," Corcoran said.

The others suffered less severe injuries, he said.

Investigators were trying Friday to determine the cause of the blast at Gerdau Ameristeel Mill in north Knoxville. Corcoran said it occurred in a part of the plant where a furnace melts down the metal.

Nearby residents said the blast could be heard from blocks away.

Ryan McClane told the Knoxville News Sentinel he was about a block away when he heard a loud blast and saw black smoke coming from the plant.

"I could feel it in my chest when it blew up," McClane said. "It sounded like a bomb going off. Nothing like I ever felt or heard in my life."

Shane Nichols told Knoxville station WATE-TV he also felt the explosion.

"It shook the whole house, I thought it busted some windows. I heard car alarms going off so when I ran outside to my front porch I saw smoke blowing out of every end of the whole factory," he said.